There is no rules or regulation about required minimum marks percentage to apply for Indian Engineering Services examination . you just pass BE or B tech engineering course. here I am giving you information about required eligibility to apply for Indian Engineering Services exam .

IES exam details :
Categories of which recruitment is made through the Examination
Civil Engineering
Mechanical Engineering
Electrical Engineering
Electronics & Tele-communication Engineering

Educational Qualifications
You have to passed A undergraduate degree in Engineering from a recognized university
Age-limits
21-30 years on 1st August of the year of Examination.

for your reference here i am providing you some of the good books IES please have a look on that